1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element, a light-emitting device, a lighting device, a display device, a display panel, and an electronic appliance.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
Light-emitting elements utilizing electroluminescence have been currently under active research and development. In a basic structure of the light-emitting element utilizing electroluminescence, a layer containing a light-emitting substance (hereinafter the layer is referred to as a “light-emitting layer”) is interposed between a pair of electrodes. By voltage application between the pair of electrodes of the light-emitting element, light can be emitted from the light-emitting substance.
Among light-emitting elements utilizing electroluminescence (EL), particularly a light-emitting element in which an organic compound is used as a light-emitting substance can be formed by stacking thin films. Because the light-emitting element can thus be reduced in thickness and weight and can have a larger area easily, it is expected to be used for a planar light source. Furthermore, the light-emitting element is expected to have emission efficiency exceeding that of an incandescent lamp or a fluorescent lamp, and thus has attracted attention as a light-emitting element suitable for lighting equipment.
Furthermore, the above light-emitting element is also expected to be used for a display panel. To obtain a display panel capable of color display (not monochrome display), there is a method in which each pixel is provided with a plurality of subpixels of different colors and the intensity of light from each subpixel is adjusted to determine the color of the pixel.
In the case where the light-emitting element utilizing electroluminescence (EL) is used for a display panel, there are the following two methods: a method of providing subpixels in a pixel with EL layers each having a function of emitting light of a different color (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters each having a function of transmitting light of a different color (hereinafter referred to as a white EL+color filter method, and it is noted that the color of the common EL layer is not limited to white).
One of the advantages of the white EL+color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer is small and no cost for patterning is incurred; thus, display panels can be manufactured at low cost with high productivity. In the separate coloring method, it is necessary to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels. However, in the white EL+color filter method, there is another advantage that the space is not necessary and therefore a high-resolution panel having higher pixel density can be achieved.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly required to be applied to a lighting and a display utilizing the white EL+color filter method.
As a light-emitting element which can emit white light, for example, a white light-emitting element in which a plurality of EL layers having emission peaks in the red, green, and blue wavelength ranges are stacked has been proposed (e.g., Patent Document 1). In addition, a white light-emitting element in which two EL layers having emission peaks in the wavelength ranges of complementary colors (e.g., blue and yellow) are stacked has been proposed (e.g., Patent Document 2). Note that such a structure in which a plurality of EL layers are stacked may be called a tandem structure.